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Our transcription: Here at the Grand Canyon, the Colorado River has cut down through the rocks showing us what's underground, but exposures like these are very rare. In most places, surface information from the geologic map is used to infer the underground distribution of rocks and structures. This is done with the geologic cross section, where the geologist has conceptually sliced the Earth open to reveal the structure of its interior. Many different cross sections can be drawn to fit the same pattern of surface exposure, but geologists recognize that the simplest one usually turns out to be the most accurate. In most cases, evidence from drilling or seismic sounding validates this assumption. The process of choosing the simplest explanation from a group of possibilities is not unique to geology. This approach is used throughout the sciences for solving different problems.
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